For use in a fuel line system of a motor vehicle or of some other vehicle various kinds of hoses are known, which can be produced from different materials. Such a hose can be used as the fuel line itself or as a connecting piece for connecting a fuel line and it can be implemented as a flexible or as an inflexible component. Normally, these lines are used for conveying petrol or diesel fuel to the engine, guaranteeing pressure compensation at and in the fuel tank, connecting the activated carbon filter to the air vent system, and regenerating said activated carbon filter by a supply of fuel.
With regard to the prevailing environmental protection and safety considerations, the pipes and hoses, respectively, have to fulfill the requirement that permeation of the conventional fuels through walls is suppressed to the highest possible degree. A measure which proved to be useful for this purpose is the arrangement of a barrier or blocking layer in the hose, which satisfies these safety requirements. Pipes which proved to be increasingly useful in this respect are pipes comprising in addition to the barrier layer also further layers, e.g. a protective layer and an adhesion promoter layer, so as to impart the necessary strength and durability to the hose.
The material used for the support layer normally consists of polyamide materials, e.g. PA 11/12. These known fuel lines proved to be disadvantageous insofar as the material used for the support layer entails high costs. This is particularly relevant as far as tank ventilation pipes are concerned, since these tank ventilation pipes have, on the one hand, comparatively large and long dimensions and are, on the other hand, only subjected to very small loads form the mechanical as well as from the chemical point of view. Since polyamides produce, in addition, only a comparatively poor blocking effect against fuels, it is obvious to use for these lines—which are located on the tank—a material, as a support material, which corresponds to that of the tank itself. PE, especially HDPE, is therefore an obvious material to use.
Furthermore, U.S. Pat. No. 5,937,911 discloses a fuel supply hose comprising an inner layer of fluorine-containing resin, an intermediate layer consisting e.g. of a polybutylene terephthalate, and an outer layer produced from a rubber material. In order to guarantee sufficient adhesive strength between the individual layers, it is here necessary to form a coherent structure of the individual layers by heat treatment and vulcanization. Another example of a pipe composed of only two layers is described in example No. 23 and table 11. The outer layer consists here of a polybutylene terephthalate and the inner layer is formed of a copolymer consisting of ethylene and tetrafluoroethylene. In this case, only a surface treatment of the inner layers has been executed in order to improve the adhesive strength. The results achieved show a very poor or a comparatively poor peel strength, which results in peeling off or in a separation of the inner and outer layers.
Also DE 42 14 383 describes the advantageous effect of polyesters, e.g. PBT, in particular their blocking effect against pure hydrocarbons as well as alcohol and, simultaneously, against mixtures of these substances, in the case of which the percentage of alcohol can vary over a wide concentration range. In comparison with the known polyamides used in fuel lines, e.g. PA11 or PA12, polyester has, however, a lower resistance to hydrolysis so that this material cannot be used as a monopipe, but can here be used as a layer in a multilayer pipe.
In DE 42 14 383 a fuel pipe is suggested, which comprises an outer layer of polyester, e.g. PBT, a subsequent adhesion promoter layer consisting of a polymer, in particular of polyurethane, and an additional layer arranged in the interior of the pipe and consisting preferably of polyamide. The adhesion promoter layer is in this respect necessary, since the adherence between polyesters and polyamides is often insufficient. In this connection, it is also possible to arrange more than one adhesion promoter layer between the inner layer and the outer layer. The polyamide layer is, however, disadvantageous insofar as this material is, on the one hand, very expensive and, on the other hand, unsatisfactory in view of the known permeation of the fuels through the wall and the resultant swelling. Although part of these disadvantages are diminished by the use of a PA layer in a multilayer hose, the surface treatments used for improving the adhesive strength between the individual layers are, however, often very complicated and expensive so that the total cost for the hose is increased substantially.